Bend-capable stent prosthesis

ABSTRACT

Normally, when stents are bent, inside the body of the stented patient, there is head-to-head collision between facing V-points on the inside of the bend. However, by alternating between two whole numbers the number of struts between successive connectors around the circumference of each of the stenting rings, the V-points are caused to veer circumferentially in opposite directions as they approach each other on the inside of the bend, so allowing them to pass by each other without collision, thereby allowing in the same stent both close packing of the ring stack, and an enhanced ability to tolerate severe bending, after placement in the body.

FIELD OF THE INVENTION

This invention relates to a stent prosthesis which is tubular and has a matrix of struts that provide a stenting action that holds bodily tissue radially away from any lumen defined by the stent matrix, around a longitudinal axis of the prosthesis. One such prosthesis is disclosed in applicant's WO 01/32102.

BACKGROUND

Currently, the great majority of stents delivered transluminally and percutaneously to a stenting site in a human body are made of a biologically compatible material which is a metal. Many stents are made of stainless steel, and many others are made of nickel titanium shape memory alloy. The nickel titanium stents are invariably self-expanding stents that utilise a shape memory effect for moving between a radially compact transluminal delivery disposition and a radially larger stenting disposition after placement in the body. Stainless steel stents are often delivered on a balloon catheter, with inflation of the balloon causing plastic deformation of the material of the struts, but other stainless steel stents rely on the resilience of the steel to spring open when a surrounding sheath is retracted relative to the stent being deployed.

However, in all cases, it is difficult to endow the stent strut matrix with a degree of flexibility that comes anywhere near the degree of flexibility of the natural bodily tissue at the stenting site. The strength and resilience of the stent matrix, that serves to push radially outwardly the bodily tissue at the stenting site, is difficult to reconcile with the flexibility in bending that the natural tissue around the stent is capable of exhibiting, in normal life of the patient carrying the stent. It is one object of the present invention to improve the performance of a stent prosthesis in bending, after it has been deployed in the body of a patient.

To explain the problem, reference will now be made to applicant's WO 01/32102, specifically drawing FIGS. 3 and 4, and the text, of WO 01/32102. Indeed, accompanying drawing FIGS. 1 and 2 are the same as FIGS. 3 and 4 of WO 01/32102.

Looking at accompanying FIG. 1, we see part of the circumference of a tubular workpiece of nickel titanium shape memory alloy, in side view. The tube has a diameter D and a multiplicity of slits 20, 22 and 24, through the wall thickness of the tubular workpiece, all parallel to each other and to the longitudinal axis of the workpiece and creating out of the original solid tubular workpiece a lattice which can be expanded radially outwardly, (for example on a mandrel) to the expanded configuration of drawing FIG. 2 (again in side view). Out of the multitude of parallel slits can now be recognised as a sequence of 10 stenting rings, all displaying a zig-zag advance around the circumference of the prosthesis. Terminal zig-zag rings 30 are composed of 24 struts 32 interspersed by points of inflection 34, giving the end view of the prosthesis the appearance of a crown with twelve points.

The eight zig-zag rings at intermediate points along the length of the stent, between the two end rings 30, are referenced 36. They are made up of struts 38 which are all much the same length, somewhat shorter than end struts 32. Between any two struts of any of the zig-zag stenting rings there is a point of inflection 40. In the two end rings 30, all twelve of these points of inflection remote from the crown end of the terminal ring 30 are connected to a corresponding point of inflection 40, head to head, in the next adjacent internal stenting ring 36. However, between any two internal stenting rings 36, not all the twelve points of inflection, found spaced around the circumference of the prosthesis, are joined to corresponding points of inflection on the next adjacent stenting ring 36. Indeed, reverting to FIG. 1, it is easy to see that there will be only four connector portions 42, linking any two adjacent internal stenting rings 36.

Thinking about advance of the prosthesis of FIG. 1, in its compact disposition, along a tortuous, transluminal, delivery path to the stenting site, as the stent bends around a sharp bend in the delivery path, on the inside of any such bend, for example at point 44 on FIG. 1, the points of inflection facing each other across the gap 60 will approach one another. Depending on the length of the diametrically opposed connector portions 42 connecting stenting rings 36B and 36C, the two unconnected points of inflection will come into contact with each other in the middle of the gap 60, in dependence upon how sharp is the bend that the stent is negotiating in the tortuous path at that time. The longer the axial gap between adjacent stenting rings, the greater the capability of the stent for negotiating ever tighter bends in the delivery path lumen.

But what of the performance of the stent in bending, after it has been deployed at the stenting site.

We can see from FIG. 2 that the pattern of connector portions 42 is symmetrical. That is to say, standing on one of these connector portions, and looking along the length of the prosthesis, the pattern of connectors to the left of the line of view is a mirror image of the pattern of connectors to the right of that line of view. If we switch to consideration of drawing FIG. 3, which shows a portion of the strut network of the stent of FIGS. 1 and 2, this is more readily evident. Just as points of inflection on the inside of a tight bend of the stent in its compact disposition of FIG. 1 can butt up against each other face to face, so can the same phenomenon occur when the expanded stent of FIG. 2 is subject to sharp bending. Any such intermittent abutment of otherwise free points of inflection is liable to have negative effects including, for example, irritation or injury to bodily tissue caught between the abutting points of inflection, or even incipient buckling of the stent with the potential to reduce flow of bodily fluid through the stent lumen to dangerously low levels.

It is one object of the present invention to mitigate these risks.

SUMMARY OF THE INVENTION

The matrix of struts of a radially expandable stent can be looked upon as a two dimensional lattice (when the tubular stent is opened out flat on a plane) and if the lattice has a regular structure (which it invariably does) then it is possible to define the lattice using a concept familiar in crystallography, namely, the “unit cell” characteristic of a space lattice of points, with each point of the space lattice corresponding to one of the connector portions in the stent matrix. Conventionally, as in the structure shown in FIGS. 1 to 3 discussed above, the unit cell is aligned with the longitudinal axis of the prosthesis. In accordance with one aspect of the present invention, however, the axially adjacent stenting rings are separated only by a small gap, and the unit cell is deliberately “skewed” with respect to the longitudinal axis of the prosthesis. This has the consequence that, when the expanded stent prosthesis is sharply bent, points of inflection that would otherwise approach each other head to head are prompted by the stresses arising in the lattice of struts to shear sideways, in opposite directions around the circumference of the stent prosthesis so that, when the tightness of the bend is finally such as to bring the points of inflection close to each other, they pass side by side rather than impact head to head.

Note that the axial gap between two radially expanded rings of a straight stent is virtually identical to the length of the gap between the same two rings in the compressed stent n the delivery catheter. But the points of inflection are much further away from the longitudinal axis, with the consequence that the amount of axial movement of facing points of inflection, for any particular degree of bending of the axis, is much greater with the stent radially expanded. A small axial gap might therefore suffice, in the delivery disposition of a stent while being inadequate to prevent head to head impact in the expanded disposition.

The small gap between axially adjacent stenting rings is important for the establishment of usefully high radially outwardly directed stenting forces. It is the tendency of the points of inflection (peaks) to pass by each other, when the stent bends, in overlapping side-by-side relationship, that opens up the possibility to keep the gap so small.

A relatively simple way to accomplish this desirable result is to arrange that, when the number of struts “N” of any stenting ring B lying between any two adjacent connector portions is such that N/2 is an even number, so that the connector portions at one axial end of ring B cannot lie circumferentially halfway between any two connector portions on the other axial end of ring B. Note that in FIG. 2 above, there are six struts of any particular stenting ring 36 between adjacent connector portions 42 on the same axial end of that stenting ring 36. Half of six is three, and three is not an even number. Proceeding from any particular connector portion 42 of the matrix of FIG. 2, it takes three struts to reach the next adjacent connector portion, whichever path one takes when departing from the base connector portion 42. In accordance with the present invention, the number of struts taken to reach the next adjacent connector portion 42 is not always the same. In consequence, the stresses imposed on the struts by bending the prosthesis sharply (into a banana shape) are going to be distributed asymmetrically with respect to any particular connecter portion 42 and it is this asymmetric stress distribution that will skew the free points of inflection relative to those facing them in the next adjacent stenting ring, so that they do not abut each other head to head on the inside of the bend of the banana shape.

Thus, in accordance with another aspect of the invention, there is provided a prosthesis that is expandable from a radially compact delivery disposition to a radially expanded stenting disposition, and is composed of a stack of zig-zag stenting rings of struts that end in points of inflection spaced around the circumference of a stenting lumen that is itself on a longitudinal axis of the stent, each of the points of inflection being located at one or the other of the two axial ends of each ring, with adjacent rings A, B, C in the stack being connected by straight connectors linking selected facing pairs of points of inflection of each two adjacent rings, circumferentially intervening pairs of facing points of inflection being unconnected, and with progress from strut to strut via the points of inflection, around the full circumference of one of the stenting rings B, namely one that is located axially between adjacent rings A and C in the stack, the connector ends encountered during such progress connect ring B alternately, first to ring A, then to ring C, then to ring A again, and so on characterised in that the connectors are parallel to the longitudinal axis and are shorter than said strut length the pairs of unconnected points of inflection remain facing, in the radially expanded disposition, for as long as the longitudinal axis remains a straight line the number of struts in ring B that lie between successive said connector ends that join ring B alternately to ring A, then ring C, is a whole number that alternates between two different values; and the connectors are so short that, when the stent functioning as a stent is caused to bend, such that the longitudinal axis becomes arcuate, the facing pairs of unconnected points of inflection that are on the inside of the bend eventually pass axially past each other, side by side, circumferentially spaced from each other, rather than impacting on each other, head to head.

A stent construction in accordance with the invention is only marginally more complex than the simple and “classic” zig-zag stenting ring construction evident from drawing FIGS. 1 to 3. The stenting rings can be a simple zig-zag construction of struts all the same length, and the connector portions can be nothing more than a plane of abutment between abutting points of inflection in adjacent zig-zag stenting rings, or simple, short, straight portions aligned with the longitudinal axis of the prosthesis. This is advantageous, when it comes to modelling the fatigue performance of the stent, something of significant importance for government regulatory authorities and for optimising stent performance long term.

There is another valuable performance enhancement that the present invention can deliver, namely attainment of full performance of any particular “theoretical” stent matrix. In reality, every placement of a stent is an individual unique event. To some extent, every stent of shape memory alloy has had its remembered shape set in a unique heat treatment step. Referring back, once again, to WO 01/32102, we set the remembered shape before removing bridges of “scrap” material between stenting rings. In consequence, remembered shapes are highly orderly and regular, much closer to the “theoretical” zig-zag shape than can be attained when the rings are only connected by a minimum of connectors during the shape-setting step. We can have this advantage also with stents in accordance with the present invention, to optimise the bending performance of the stents, and the fatigue resistance that comes from having stress distributions close to optimal, every time.

For a clear understanding of the invention definitions are useful for “strut length” and “connector length”. Fortunately, such definitions are more or less self-evident, after consideration of how stents are made.

Normally, one begins with a tubular workpiece and creates in it a multitude of slits that extend through the wall thickness. They have their length direction more or less lengthwise along the tube. Circumferentially, adjacent slits are axially staggered. This is not unlike the way of making a simple “expanded metal” sheet having diamond-shaped apertures, familiar to structural engineers, and those who clad dangerous machinery in see-through metal sheet material to serve as safety guards.

For stent making, a useful extra step is to remove many of the residual links between adjacent diamonds. See again WO 01/32102, mentioned above.

The slit creation step can be by a chemical process such as etching or a physical process such as laser cutting. For nickel titanium shape memory alloys, the usual method is laser cutting.

So, now, how to define strut length and connector length? These lengths emerge quite simply from an inspection of the axial lengths by which circumferentially adjacent slits overlap. For a strut length one would measure axially from the end of one slit (that is defining one of the two flanks surfaces of the strut under consideration) to the end of the circumferentially next adjacent slit that has, as one of its defining long walls, the other flank surface of the strut whose length is to be ascertained. This method yields relatively short lengths. It is as if one were a tailor, and were to measure arm length from the armpit rather than from a point on top of the shoulder of the person being fitted.

The same logic applies when determining connector lengths. They correspond to the length of the gap that is created, when material is moved from the stent workpiece, in the unslitted material between two co-linear slits through the wall of the workpiece, said removal of material revealing two axially facing points of inflection when the stent matrix is subject to radial expansion. Thus, in the limiting case, the connector length is the same as the width of the laser beam that removed material to create that gap. Again, see WO 01/32102 mentioned above.

Of course, connector lengths and strut lengths can vary over the stent. Some of its stenting rings may have longer struts than others. However, except for very special cases, a stent is indifferent to rotation about is long axis, so that changes in the rotational orientation of the stent relative to the bodily lumen being stented, during advancement of the stent along the lumen to the stenting site, do not render the stent unfit for placement. Thus, for purposes of clarity in the here-claimed invention, it will always be possible to divine clearly a strut length and a connector length, for testing whether the definition of the invention is met in any particular zone of a stent that corresponds to two adjacent stenting rings and the gap in between.

With the published state of the art there are disclosures, such as in US2004/0117002 and US 2003/0225448, of stents composed of zig-zag stenting rings with straight connectors that join adjacent stenting rings peak-to-peak and with alternating whole numbers of struts lying between circumferentially adjacent connectors terminating in any one ring of such struts. Such stents exhibit face to face (otherwise here called “peak to peak”) facing points of inflection in the radially compact pre-expanded disposition of the stent. Such stents are relatively easy to make by laser cutting of a precursor tube of raw material. Whether such stents still exhibit face to face points of inflection after expansion is unclear. What happens when the stents bend is also unclear. What is clear is that the writers of these prior publications did not include any teaching about how facing points of inflecting may tend to move in opposite circumferential directions on bending of the stent, and thereby ease away from head to head collision. A failure to recognise this phenomenon results in a failure to appreciate the scope to reduce the length of the connectors connecting adjacent stenting rings, thereby missing a chance to maximise radially stenting force and strut coverage of the wall of the bodily lumen that has been stented.

The disclosure of WO98/20810 is instructive. It describes laser cut stents of nickel titanium shape memory alloy, with zig-zag stenting rings that expand to a stenting diameter. It teaches that the straight connectors linking axially adjacent stenting rings are to be at a slant to the longitudinal axis so that what would otherwise be the facing points of the “V-shaped segments” are circumferentially staggered, to minimise contact between these peaks when the expanded stent is bent such that the longitudinal axis becomes arcuate. Another reason for staggering the V-points around the circumferences is to improve the homogeneity of coverage of the lumen wall with the strut matrix of the stent, to leave no zones of coverage of the lumen wall tissue that are more sparse than other zones. The connectors shown in the drawings do appear to be quite long and it is of course self-evident that, the longer the connectors, the longer are the gaps between axially adjacent zig-zag rings, such gaps corresponding to sparse coverage of the lumen wall bodily tissue in the zones of tissue in the gaps between the rings. In other words, the shorter the connectors, the less need there is to stagger the V-point peaks circumferentially, in order to maintain lumen wall coverage by the matrix of struts of the stent.

When assimilating the disclosure value of WO98/20810 it is instructive to imagine the stent in radially fully expanded disposition. The circumferential arc between two points of inflection is multiple times more than in the radially compressed delivery, and multiple times more than the circumferential distance between the opposite ends of a slanting connector. This has the consequence that the degree by which peak to peak impact is alleviated, by a short slanting connector, is disappointingly small, and gets relatively smaller with every increase in diameter of the expanded stent. By contrast, with the present invention, the greater the diameter of the expanded stent, the more powerful the effect to circumferentially stagger the points of inflection.

It will be evident to the skilled reader that the term “stenting ring” can be understood also to include in its scope successive turns around a stent lumen of a spiral that is composed of struts in a zig-zag arrangement which spiral advances along the stent lumen away form one of the stent and towards the other.

Struts need not be of constant cross-section. Indeed, for optimisation of stress distribution within the struts, and hence of the fatigue performance of the stent the cross-section will indeed change, along the length of each strut. The struts need not all be the same as each other. There could be different strut species, either from ring to ring or, indeed, within a stenting ring. A common arrangement is to have rings of longer struts at each end of the stent, the shorter struts at a mid-length portion, providing greater radially outward stenting force.

The stent can be a bare stent or a covered stent such as a stent graft. The stent may be a drug-eluting stent. The stent may have a function other than to hold a bodily lumen open against stenosis. For example, the stent could be part of a filter device for temporary placement in a bodily lumen, or an anchor for some other device that is to perform a therapeutic function within a bodily lumen.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show more clearly how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:

FIGS. 1 and 2 are side views of the stent described in WO 01/32102, with FIG. 1 in the compact delivery disposition and FIG. 2 in the radially expanded deployed disposition of the prosthesis.

FIG. 3 is a diagram of symmetrical matrix of connector portions (not unlike the embodiment of FIGS. 1 and 2), opened out flat on a plane, and

FIG. 4 is a diagram corresponding to that of FIG. 3, but with a matrix of struts and connectors in accordance with the present invention

FIG. 5 is a photographic side view of a stent prosthesis which exhibits the strut and connector matrix of FIG. 4, expanded but not subject to any bending stresses; and

FIG. 6 is a photographic side view of part of the stent prosthesis of FIG. 5, but bent into a “banana” shape to reveal how the points of inflection move relative to each other and relative to the addressed unbent configuration of FIG. 5.

DETAILED DESCRIPTION

What is shown in FIGS. 1, 2 has been described above and in applicant's earlier WO 01/32102. The reader is referred to the passages above and to the prior publication.

FIG. 3 is not unlike the embodiment of FIGS. 1 and 2, but the length of the elongate connectors 42 helps to reveal the pattern of connectors in the lattice.

FIG. 4 reveals a matrix of struts 38 and connectors 42 spacing apart a succession of zig-zag stenting rings 36 (four are visible in FIG. 4). Starting from connector 42A, we can reach adjacent connector 42B via a sequence of three struts 38ABC. But not all adjacent connectors are as close. Consider adjacent connector 42C. It takes five struts, namely struts 38D to H, to reach connector 42C. The pattern is repeated throughout the matrix. Note that the connector 42D that links zig-zag rings 36C and 36D is displaced circumferentially sideways from connector 42A, unlike the arrangement in FIG. 3. If we imagine in FIG. 4 connectors 42A and 42D lying on the inside of a severe bend of the expanded stent matrix, so that the points of inflection 40X on zig-zag ring 36B, and the points of inflection 40Y on zig-zag ring 36C, are moving towards each other, the stresses imposed by connector 42A on stenting ring 36B and those imposed on stenting ring 36C by connector 42D will be unsymmetrical. It does not require a great exercise of imagination to visualise points of inflection 40× and points of inflection 40Y failing to meet each other face to face when the bend is tight enough but, instead, sliding past each other, with spacing.

Turning to drawing FIGS. 5 and 6, we see occurring in practice exactly what one can, with a degree of imagination, visualise occurring from the diagram of FIG. 4. Whereas the free points of inflection in FIG. 5, the unstressed configuration of the expanded stent, are bravely facing each other without any circumferential staggering, as soon as the prosthesis is subject to external stresses that bend it into the banana shape evident from FIG. 6, what was previously and orderly face to face configuration of points of inflection has now become a staggered configuration, not just on the exact inside of the bend but also on the flanks of the bend that are facing the viewer in the side view of FIG. 6.

Self-evidently, the construction of FIG. 5 is hardly more complex than that of FIG. 2. Likewise, the construction of FIG. 4 is self-evidently hardly more complicated than the FIG. 3 matrix. It is one advantage of the present invention that the useful result evident in FIG. 6 can be achieved with a lattice that is barely more complicated than that of the classic lattice of WO 01/32102. That is of course not to say that the benefits of the invention are not achievable with more complicated constructions. There is now an enormous multitude of stent lattice possibilities and those who are promulgating relatively complicated lattices would doubtless assert that their specific constructions bring useful benefits. Doubtless the simple principle of the present invention can be incorporated into these more complicated arrangements, as skilled and experienced stent design readers will appreciate.

As increasing sophistication of design of stents allows them to perform in ever more demanding locations in the body, the need for stent flexibility in bending continues to increase. for maximum flexibility, one would wish for a minimum of connector portions between stenting rings. However, the point about connectors is that they do serve to keep apart from each other portions of stenting rings that might otherwise collide. There is therefore a tension between the objective of preventing collisions and the objective of greater flexibility. The present invention aims to make a contribution to this delicate contradiction, by using just a few connectors to encourage approaching points of inflection to, as it were, politely step to one side, in opposite directions, as they approach each other, rather than confronting each other head to head. Given the strength that effective stents need to exhibit, to keep bodily tissue displaced radially outwardly from the bodily lumen being stented, there should be enough strength in even just a few connectors to ease the points of inflection past each other, because only a relatively small “push” on the points of inflection, in circumferentially opposite directions, should be enough to prevent a peak-to-peak confrontation. Otherwise, when the stent in the body is not called upon to bend, then the connectors do not have to go to work to urge the facing points of inflection to move in opposite circumferential directions. The stresses in the stent matrix are those that arise anyway, when the surrounding tissue is urging the stent matrix to bend from a straight tube to a banana shape. Accordingly, the stresses within the stent matrix are in harmony with the stresses that the surrounding body tissue is experiencing, and imposing on the stent. This harmony should be of assistance in matching the performance of the metal stent matrix to the resilient properties of the surrounding bodily tissue.

There is no requirement that the skewed arrangement, that the present invention proposes, be reproduced throughout the stent lattice. For example, it may be desirable to make one portion of a stent more bend-capable than other parts. In such a case, it may be useful to confine the skewed connector distribution to those parts of the stent which are to be relatively more bend-capable. It hardly needs to be observed that the bend capability of a stent portion, before it begins to buckle, should be high enough to incur the risk of abutment of approaching points of inflection in adjacent stenting rings, to make incorporation of the skewed distribution of the invention worthwhile. Generally, the sparser the population of connector portions between the population of connector portions between stenting rings, the more bend-capability will be available.

FIG. 3 shows 6 struts between adjacent connectors in the same circle, and FIG. 4 shows 8. With 10 connectors, an unsymmetrical arrangement of the present invention suggests a heavily skewed split of 3/7 in the number of struts between each connector and the nearest one in the axially next adjacent ring of connectors (with the symmetrical arrangement being 5/5). 12 connectors seem scarcely more attractive because then the split is 4/8, still somewhat heavily skewed relative to a symmetrical 6/6 split of struts between connectors, but 14 connectors seems more attractive because that permits a 6/8 split which is close to the symmetrical 7/7 split of a symmetrical arrangement. One seeks an arrangement that is skewed enough to urge the approaching points of inflection on the inside of the bend to pass each other elegantly, but not such a pronounced skew that stresses in the stent lattice show pronounced differences, depending where in the lattice one is measuring them.

Generally, there will be up to 6 connectors in each circle of connectors. 3 or 4 connectors per ring are presently favoured but the number of connectors falls to be determined in harmony with many other design aspects of the stent lattice, as stent designers well know.

The radially outwardly directed force that a stent can exert against the bodily tissue forming the walls of the stented bodily lumen will inevitably be somewhat reduced, with increasing length of the gaps between axially adjacent stenting rings of the stent. Clearly then, one would choose short connectors to maximise stenting radial force. In a high flex location for the stent measures must be taken, to prevent collisions between adjacent stenting rings when the stent is subjected to serve bending. A particularly useful technical effect of the present invention is that the short connector portions allow close proximity of axially adjacent stenting rings (and so a high stenting force) yet no collisions between the closely adjacent rings when the stent suffers severe bending.

EXAMPLE

To assist readers to grasp the physical dimensions of stents that are preferred embodiments of the present invention, we set out in the Table below some representative dimensions for stents studied by the Applicant. It is to be understood that these dimensions are provided not to signify precise dimensions that work better than others but merely dimensions within the ranges here contemplated.

TABLE Each zig-zag ring Connector Strut Strut extended Number of width length Connector length Product struts (μm) (mm) (mm) length (mm)* A 24 160 1.95 0.8 1.4 B 36 100 1.45 0.5 1.0 C 30 100 1.45 0.5 1.0 D 32 135 1.55 0.5 1.0 *This is the full length that lies between the ends of two co-linear slits axially spaced from each other that create the two axially-facing V-points of inflection of two adjacent zig-zag rings

One message to be taken from the Table is that strut lengths are going to be, in general, significantly more than 1 mm while connectors are going to exhibit a length significantly below 1 mm. The points of inflection, in themselves, typically have an axial length of 0.25 mm or 0.30 mm, which is typically around two or three times the width (in the circumferential direction) of one of the struts. Thinking of a point of inflection as a zone where the material of two struts comes together in an unslitted block of material, that block will have the width of two struts and an axial length that is similar to, or a bit longer than, such width.

In general, connectors lengths will be 0.8 mm or less, likely 0.6 mm or less. Strut lengths will likely be more than 1.25 mm, likely is a range of from 1.3 to 2.2 mm or more specifically 1.4 to 2.0 mm. One favoured construction has 32 struts per ring, such as in Product D in the Table.

For the sake of clarity, and the avoidance of doubt, the “points of inflection” referred to in this specification are not a reference to the point of inflection that each strut exhibits, mid-way along its length, which more or less inevitably appears when the slitted stent precursor tube is radially expanded form its original diameter to its working stenting diameter. 

1. A prosthesis that is expandable from a radially compact delivery disposition to a radially expanded stenting disposition, and is composed of a stack of zig-zag stenting rings of struts that end in points of inflection spaced around the circumference of a stenting lumen that is itself on a longitudinal axis of the stent, each of the points of inflection being located at one or the other of the two axial ends of each ring, with adjacent rings A, B, C in the stack being connected by straight connectors linking selected facing pairs of points of inflection of each two adjacent rings, circumferentially intervening pairs of facing points of inflection being unconnected, and with progress from strut to strut via the points of inflection, around the full circumference of one of the stenting rings B, namely one that is located axially between adjacent rings A and C in the stack, the connector ends encountered during such progress connect ring B alternately, first to ring A, then to ring C, then to ring A again, and so on: characterised in that the connectors are parallel to the longitudinal axis and are shorter than said strut length; the pairs of unconnected points of inflection remain facing, in the radially expanded disposition, for as long as the longitudinal axis remains a straight line; the number of struts in ring B that lie between successive said connector ends that join ring B alternately to ring A, then ring C, is a whole number that alternates between two different values; and the connectors are so short that, when the stent functioning as a stent is caused to bend, such that the longitudinal axis becomes arcuate, the facing pairs of unconnected points of inflection that are on the inside of the bend eventually pass axially past each other, side by side, circumferentially spaced from each other, rather than impacting on each other, head to head.
 2. The prosthesis as claimed in claim 1, wherein the number of struts “N” of ring B lying between any two adjacent A-B connector portions is such that N/2 is an even number, so that the B-C connectors cannot be placed circumferentially on ring B half way between any two of the A-B connectors.
 3. The prosthesis as claimed in claim 1, wherein the number of struts “N” of ring B lying between any two adjacent A-B connector portions is such that N/2 is an odd number and N is more than
 10. 4. The prosthesis as claimed in claim 1, wherein the lattice points of the connectors together exhibit a helical path that is coaxial with the longitudinal axis of the prosthesis.
 5. The prosthesis as claimed in claim 1, wherein each of the stenting rings is composed of struts that have all the same length, such that each of the points of inflection in such a ring lie in one of two circles transverse to the longitudinal axis of the prosthesis.
 6. The prosthesis as claimed in claim 1, wherein all the struts in rings A, B and C have the same cross-section and length.
 7. The prosthesis as claimed in claim 1, comprising a shape memory alloy.
 8. The prosthesis as claimed in claim 1, which undergoes plastic deformation expansion to its stenting disposition.
 9. The prosthesis as claimed in claim 1, which is a peripheral vascular stent.
 10. The prosthesis as claimed in claim 1, which is a biliary stent.
 11. The prosthesis as claimed in claim 1, wherein the connectors in rings A, B, C have a length which is less than 1 mm.
 12. The prosthesis as claimed in claim 11, which connector length is not more than 0.8 mm.
 13. The prosthesis as claimed in claim 12, which connector length is less than 0.6 mm.
 14. The prosthesis as claimed in claim 1, with a strut length of more than 1.25 mm within the rings A, B, C.
 15. The prosthesis as claimed in claim 14, with a strut length in a range of from 1.3 mm to 2.2 mm
 16. The prosthesis as claimed in claim 15, with a strut length in a range of from 1.4 mm to 2.0 mm.
 17. The prosthesis as claimed in claim 1, with 32 struts in each of the zig-zag rings A, B, C.
 18. The prosthesis as claimed in claim 1, with 4 evenly spaced connectors between ring A and ring B. 